Argos joined a growing band of retailers making plans to desert Royal Mail today as the Communication Workers Union voted overwhelmingly in favour of a national strike over proposed changes to working conditions.

The catalogue group said it had put contingency arrangements in place with rival operators to ensure its deliveries were not disrupted by the strikes.

The Guardian has also learned that a group of 30 online retailers, meeting at a trade fair at Earl's Court in London, have agreed to switch to other postal operators because of the disruption at Royal Mail.

Jonathan DeCarteret from Post-Switch, a consultant present at the meeting, said: "A large number of these companies feel that they can no longer trust Royal Mail to deliver to their customers. Deals are being struck left, right and centre with its rivals. We find that once a company leaves Royal Mail they never come back."

Post-Switch estimates that one quarter of Royal Mail's 100 biggest customers, including banks, utilities and charities, were now talking to rivals to switch business. "The vote yesterday is only speeding up this process," DeCarteret added.

Post-Switch said last month it handled almost three times as many inquiries from frms wanting to find an alternative to Royal Mail than usual, after months of regional stoppages at branches.

In a further dent to Royal Mail's fast- shrinking customer base, eBay also said it was talking to other companies to make sure deliveries ran smoothly in the runup to Christmas.

Separately a survey of 250 businesses, by the British Chambers of Commerce, found three-quarters were now considering using another delivery service.

Yesterday the Guardian revealed that the online retailer Amazon, the Royal Mail's second largest customer, had already discussed moving its £25m contract to deliver packets larger than 500g to the rival Home Delivery Network.

Amazon confirmed it was "working on contingency measures with other carriers" but disputed the characterisation of its HDN deal as a cancellation of a long-term contract. Industry sources said it came several weeks before a regular break-clause in Amazon's contract with Royal Mail, causing concern for the state-owned group and jubiliation at HDN.

Yesterday Royal Mail and the Communication Workers Union (CWU), which organised the ballot, traded thinly veiled insults as industrial relations sunk to a new low. The CWU called the state-owned group's management, led by the former Football Association head Adam Crozier, "arrogant", and said the 3-1 vote in favour of industrial action represented a "damning verdict" on the way the postal service was run. Royal Mail hit back, condemning the strike as "reprehensible" and warning it would be "very damaging".

Talks today between the union and Royal Mail could avert a strike. CWU leaders will meet Monday to discuss whether to issue notice of national action. A spokeswoman said: "The ball is in Royal Mail's court."

The union claims Royal Mail is pushing through plans to modernise the business too quickly and without consultation. The postal group is more inefficient than rivals, and mail still has to be sorted by hand before any post round. Royal Mail's plans would alter the nature of the job, as well as hours, for many workers.

The CWU leader, Billy Hayes, accused the government and Royal Mail of refusing to negotiate with the workforce over modernisation. The union's deputy leader, Dave Ward, said: "We've seen cuts and increased workloads and now we need an agreed rollout of real modernisation. Aligning the interests of customers, employees and the company as a whole is a prerequisite for the successful modernisation of Royal Mail."

Mark Higson, Royal Mail's managing director, said the firm had had more than 70 meetings with CWU in recent months . He added that Royal Mail was not planning to introduce any more big elements of the modernisation scheme this year.

Alternative methods

Robert Hammond, a post expert at Consumer Focus, warned a national strike in the runup to Christmas "will lead to a miserable time for consumers and businesses alike". But there are ways of mitigating the misery.

Consumers should not rely on postal deliveries when waiting for bills or paying them, but find an alternative method such as phone, online or at a bank or post office, Hammond advised. For businesses who need packages delivered urgently, eschewing "snail mail" for courier companies is another, more expensive option.

But for ordinary consumers planning on doing their Christmas shopping online, it is impossible to avoid the effects of strike action altogether. It is true that online retailers like Amazon and Argos are talking to other postal operators about delivering medium-sized packages like box sets. But often these packages end up at the "last mile" of Royal Mail's network – its local sorting offices – which rely on its postmen to deliver it. The best advice for those sending Christmas cards is to post them early – really early.